An imaging sensor is a device that is used to convert an optical image to electrical signals. Currently used types of imaging sensors include complementary metal-oxide-semiconductor (CMOS) imaging sensors and the semiconductor charge-coupled device (CCD) imaging sensors, etc. The benefits of CCD imaging sensors include high sensitivity and low noise, etc. However, it may be difficult to integrate CCD imaging sensors with other devices. Further, the power consumption of CCD imaging sensors may be relatively high. Comparing with CCD imaging sensors, CMOS imaging sensors have the benefits of simple fabrication process, easy integration with other devices, small size, low weight, low power consumption and low cost, etc. Currently, CMOS imaging sensors are widely used in the static digital cameras, cell phone cameras, digital video cameras, medical video instrument (such as gastroscopes, etc.), and automobile video instrument, etc.
The basic unit of a CMOS imaging sensor is referred as a pixel. The pixel includes a photodiode and three transfer transistors or four transfer transistors, which are referred as a 3T model or a 4T model, respectively. The majority of the CMOS imaging sensors in the markets are the 4T model.
FIG. 1 illustrates a 4T model CMOS imaging sensor. The CMOS imaging sensor includes four transfer transistors, a floating diffusion region FD and one photodiode PD. The four transfer transistors are a reset transistor M1, an amplify transistor M2, a select transistor M3 and a transfer transistor M4.
Before receiving the incident light, the reset transistor M1 and the transfer transistor M4 are turned on; and the amplify transistor M2 and the select transistor M3 are turned off. The floating diffusion region FD and the photodiode PD are reset. Then, all the transistors are turned off; the photodiode receives the incident light and converts the incident light to photo-induced carriers. Then, the transfer transistor M4 is turned on; and other transistors are still turned off, the photo-induced carriers are transferred from the photodiode PD to the floating diffusion region FD. Next, the amplify transistor M2 and the select transistor M3 are turned on, the photo-induced carriers are output from the floating diffusion region FD through the amplify transistor M2 and the select transistor M3. Thus, a cycle of light receiving and transferring are completed.
The transferring of the photo-reduced carriers from the photodiode PD to the floating diffusion region FD depends the potential difference between the photodiode PD and the floating diffusion region FD. When the potential difference is greater than the potential barrier between the photodiode PD and the floating diffusion region FD, the photo-induced carriers are transferred to the floating diffusion region FD.
FIG. 2 illustrates the pixel structure of an existing N-type CMOS imaging sensor. As shown in FIG. 2, the pixel structure of the N-type CMOS imaging sensor include a P-type semiconductor substrate 101 and a transfer transistor 103 formed on the P-type semiconductor substrate 101. The transfer transistor 103 includes a gate structure (not labeled) formed on the P-type semiconductor substrate 101 and a photodiode (not labeled) formed in the P-type semiconductor substrate 101 at one side of the gate structure. The photodiode includes an N-type heavily-doped region 104 formed in the P-type semiconductor substrate 101. The N-type heavily doped region 104 is configured as a photo sensitive region. The N-type heavily doped region 104 is also configured as an anode of the photodiode. The pixel structure of the N-type CMOS imaging sensor also includes an N-type floating diffusion region 150 formed in the P-type semiconductor substrate 101 at the other side of the gate structure. Further, the pixel structure of the N-type CMOS imaging sensor includes shallow trench isolation (STI) structures 102 configured to isolate adjacent active regions.
However, it may be easy for the above pixel structure of the N-type CMOS imaging sensor to generate a dark current, thus the imaging quality of the pixel structure may be affected. The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems.